1. Field of the Invention
The present invention relates in general to a sheet conveying apparatus provided in an image forming apparatus, and more particularly to a construction for correcting a skew of a sheet being conveyed.
2. Related Background Art
Heretofore, an image forming apparatus or an image reading apparatus such as a copying machine, a printer, a facsimile or a scanner is provided with a sheet conveying apparatus for conveying a sheet such as a recording paper or an original to an image forming portion or an image recording portion. Then, some of the sheet conveying apparatuses includes correction means for carrying out skew conveying correction for a sheet and position shift correction for a sheet in order to adjust an attitude and a position of the sheet before the sheet is conveyed to the image forming portion or the image recording portion.
Here, as for a correction method for such correction means, there is one using a registration roller pair. In case of the image forming apparatus for example, there has become the main stream a so-called a loop registration method in which a leading edge of a sheet is brought into contact with a nip of the stopped registration roller pair to bend the sheet, skew conveying correction is carried out with the leading edge of the sheet being made parallel to a roller nip by elasticity of the sheet, and then the registration roller pair is rotated at a predetermined timing to adjust the synchronization between the sheet and an image.
However, in such a loop registration method, a loop space for formation of the loop is necessarily required to scale up the apparatus. In addition, when a sufficient loop space cannot be ensured, in particular, there is encountered a problem in that jam (paper plugging) due to the buckling is generated in a sheet having weak stiffness such as a thin paper, and when the sheet is brought into contact with the registration roller pair, a noise (so-called a loop noise) is generated.
Moreover, there is encountered a problem in that an ability to correct the skew conveying is changed by the strength of the stiffness of the sheet. More specifically, in case of a thin paper having weak stiffness, the abutting pressure when the leading edge of the sheet is brought into contact with the registration roller nip becomes insufficient, and hence the leading edge of the sheet cannot be sufficiently brought into contact with the registration roller pair in some cases. In such cases, the skew conveying correction cannot be perfectly carried out.
In addition, in case of a sheet having large stiffness such as a thick paper, there is nonconformity that the sheet goes through the nip of the registration roller pair due to a shock caused when the leading edge of the sheet is brought into contact with the nip of the registration roller pair. Then, if a load or the like is tried to be applied to the registration roller pair by a brake member for example in order to prevent this nonconformity, this causes cost-up of a product.
Furthermore, in a case or the like where a leading edge of a sheet is curled or folded, the leading edge of the sheet cannot be made accurately parallel to the nip portion of the registration roller pair. As a result, there is also encountered a problem in that the skew conveying correction cannot be accurately carried out to reduce the printing accuracy.
On the other hand, in recent years, the digitization for the image forming apparatus and the image reading apparatus have been realized, which leads to that after image information of an original is read once, the image information can be electrically encoded to be stored in a memory. Then, when an image is formed, the image information stored in the memory is read out to form an image corresponding to the image information of the original on a photosensitive member using a laser beam, or an exposing apparatus including an LED array and the like. Thus, even in copying for a plurality of sheets of papers, a mechanical motion as in an optical device becomes unnecessary.
As a result, a sheet interval, which is a gap between sheet and sheet, can be reduced, and hence it becomes possible to process a large number of sheets for a short period of time. As a result, in case of the image forming apparatus for example, it has become possible to realize the increasing of a substantial image formation speed in forming an image without increasing a process speed.
However, when the apparatus adopting the above-mentioned loop registration method is used as the sheet conveying apparatus, a sheet is momentarily stopped for the purpose of forming a loop. Thus, the sheet interval is necessarily determined, which will exert a large influence on the increasing of the image formation speed (productivity).
Then, a sheet conveying apparatus adopting the loop registration method for enabling the skew conveying of a sheet to be automatically corrected in order to overcome such nonconformity was proposed in Japanese Patent Application Laid-Open No. H3-67838.
Here, this sheet conveying apparatus includes a conveying roller pair (registration roller) for conveying a sheet with the sheet being held between them, a sensor provided downstream with respect to the conveying roller pair in a conveying direction for detecting the skew amount of sheet, and conveying roller skew correcting means for displacing the conveying roller pair so as to incline the conveying roller pair in a direction intersecting perpendicularly the sheet conveying direction. When the skew conveying of the sheet is corrected, the conveying roller pair is displaced so as to correspond to the skew of the sheet to thereby correct the skew conveying of the sheet on the basis of the information from the skew amount detection sensor.
However, in the conventional sheet conveying apparatus for displacing such the conveying roller pair to correct the skew conveying of the sheet, when the skew conveying of the sheet is corrected, a pivotal movement center about which the conveying roller pair is pivotally moved exists at a shaft end of the conveying roller pair.
Here, when as described above, the pivotal movement center exists at the shaft end of the conveying roller pair, there is nonconformity that if the conveying roller pair is pivotally moved about the pivotal movement center, since a point at which the skew amount of sheet is detected is largely moved along with the pivotal movement of the conveying roller pair, the skew of the sheet cannot be accurately corrected.
Moreover, in particular, in the image forming apparatus for writing an image on the basis of a signal from a sensor, there is nonconformity that if the point at which the (the skew amount of) sheet is detected is largely shifted in such a manner, a position where an image is written to the sheet is shifted accordingly, and hence the suitable image formation cannot be carried out.